Electroluminescent body

ABSTRACT

An electroluminescent component ( 1 ), in particular an LED chip, which has a high external efficiency in conjunction with a simple construction. The electroluminescent component ( 1 ) has a substrate ( 2 ); a plurality of radiation decoupling elements arranged at a distance next to one another on the substrate ( 2 ) and having an active layer stack ( 7 ) with an emission zone ( 8 ); and a contact element ( 9 ) on each radiation decoupling element ( 4 ). The contact elements ( 9 ), whose width (b′) is dimensioned such that it is less than the width (b) of the radiation decoupling elements ( 4 ), are arranged centrally on the radiation decoupling elements ( 4 ), and the width (b) of the radiation decoupling elements ( 4 ), for a given height (h), is chosen to be so small that a substantial proportion of the light ( 11 ) radiated laterally from the emission zone ( 8 ) can be decoupled directly through the side areas ( 12 ) of the radiation decoupling elements ( 4 ).

The present invention relates to an electroluminescent body, inparticular an LED chip, according to the preamble of claim 1. It relatesin particular to an LED chip in which an active layer stack hassemiconductor material based on Al_(x)Ga_(y)In_(1-x-y)N where 0≦x≦1,0≦y≦1 and x+y≦1.

Conventional LED chips usually have a single active layer stackextending over the entire growth area of a substrate. For the purpose ofinjecting current, such an LED chip has a so-called bonding pad at thefront side of the LED chip and a whole-area contact metallization isapplied at the rear side of the substrate, it being endeavored to expandthe current flow through the LED chip as far as possible to the entirelateral extent of the active layer stack.

As an alternative to injecting current vertically, in the case of whichthe active layer stack is arranged in a sandwich-like manner between twocontacts, there are also chip structures in which the contact connectionboth of the p-type side and of the n-type side is effected from thefront side of the chip. This is usually the case when the substrate forthe active layer stack is electrically insulated.

Despite a high efficiency of the light generating process in the activelayer of up to almost 100%, such LED chips have relatively low externalefficiencies. The difficulty consists in decoupling the light which isgenerated in the flat active semiconductor layers having a highrefractive index into the potting material having a significantly lowerrefractive index. In this case, it is usually only the primary lightgenerated at a relatively small solid angle which passes toward theoutside; the rest of the light is reflected back into the semiconductorby total reflection at the boundary between the semiconductor and asurrounding casting and a large part of said light is lost there throughabsorption in the active layer, in the substrate, at the substratesurface and at the electrical contact elements or the bonding pad.

An electroluminescent component with improved decoupling of light isknown from DE 199 11 717 A1 for example. The monolithicelectroluminescent component disclosed in this document has a substrate,on which are provided a multiplicity of radiation decoupling elementsarranged next to one another with respect to the main radiatingdirection of the component. The radiation decoupling elements, which arepreferably formed in cylindrical fashion, have an active layer sequencewith an emission zone with at least one electroluminescent pn junction,downstream of which are arranged a so-called current aperture layer witha current passage opening for delimiting the emission zone and a contactlayer. Annular contact elements are provided on the contact layers ofthe cylindrical radiation decoupling elements, said contact elementsbeing interconnected by electrically conductive webs. These annularcontacts cover only that region of the top side of the radiationdecoupling elements through which only little radiation, or no radiationat all, would be decoupled anyway on account of total reflection at theinterface between radiation decoupling element and the surroundingmedium.

Taking this prior art as a departure point, it is an object of thepresent invention to provide an electro-luminescent component whichensures a high efficiency of the decoupling of light in conjunction witha construction that is as simple as possible.

This object is achieved by means of an electroluminescent componenthaving the features of claim 1. Subclaims 2 to 15 relate to advantageousrefinements and developments of the invention.

The electroluminescent component has a substrate; a plurality ofradiation decoupling elements arranged at a distance next to one anotheron the substrate and having an active layer stack with an emission layerwith a laterally delimited emission zone; and a contact element on eachradiation decoupling element. Preferably, the contact elements arearranged centrally on the radiation decoupling elements and have a widthor a diameter less than the width or the diameter of the radiationdecoupling elements. Moreover, the width or the diameter of theradiation decoupling element, for a given height of the radiationdecoupling element, is chosen to be so small that a substantialproportion of the light radiated laterally from the emission zone can bedecoupled directly through the side areas of the radiation decouplingelements.

The lateral extent of the emission zone essentially corresponds to thelateral extent of the contact element. This is achieved, for a lowmobility of the charge carriers in the active layer between emissionzone and contact element, as is the case for example for p-dopedAl_(x)Ga_(y)In_(1-x-y)N where 0≦x≦1, 0≦y≦1 and x+y≦1, preferably byvirtue of the fact that the current spreading in this layer is so lowthat light is generated only in a narrow lateral region below thecontact element. Reflection losses within the active layer stack areavoided to the greatest possible extent by virtue of the invention'sdimensioning of the radiation decoupling elements, since a maximumproportion of the light radiated laterally can be decoupled directlythrough the side area of the radiation decoupling elements.

The radiation decoupling elements preferably have a strip-like structurehaving the above-mentioned width or a point-like structure having theabove-mentioned diameter. Hereafter, for the sake of simplicity, mentionis made exclusively of the width of the radiation decoupling element andof the width of the contact element, the diameter thereof being meantthereby in the case of a point-like structure of the radiationdecoupling element and/or of the contact element.

Particularly if the emission zone of the active layer stack does not liedirectly below the semiconductor surface, the radiation decouplingelements are preferably formed conically, their side facing thesubstrate having a larger cross-sectional area than their side facingthe contact elements.

The dimensions of the radiation decoupling elements preferably satisfythe condition0<(b+b′)/h<2 ct(α_(T))where b is the width of the radiation decoupling elements, b′ is thewidth of the contact elements, h is the height of the radiationdecoupling elements and α_(T) is the critical angle of total reflectionfor the light emerging from the active layer stack into the surroundingmedium.

For the case where the emission zone of the active layer stack in theradiation decoupling elements is positioned between the side facing thesubstrate and the side facing the contact elements, i.e. in particularnot directly at the area facing the substrate or at the area facing thecontact elements, the dimensions of the radiation decoupling elementsadvantageously satisfy the condition0<(b+b′)/h<cot(α)where b is the width of the radiation decoupling elements, b′ is thewidth of the contact elements, h is the height of the radiationdecoupling elements and α_(T) is the critical angle of total reflectionfor the light emerging from the active layer stack into the surroundingmedium.

Further advantageous features, advantages and expediencies of theinvention are explained in more detail below using various preferredexemplary embodiments with reference to the accompanying drawings, inwhich:

FIG. 1 shows a diagrammatic sectional illustration of a first exemplaryembodiment of an electroluminescent component according to the presentinvention;

FIGS. 2A to 2C show diagrammatic sectional illustrations of differentradiation decoupling elements such as can be used in theelectroluminescent component of FIG. 1;

FIG. 3 shows a diagrammatic sectional illustration of a further variantof a radiation decoupling element such as can be used in theelectroluminescent component of FIG. 1;

FIGS. 4A and B respectively show a diagrammatic illustration of a planview of an electroluminescent component according to the presentinvention with radiation decoupling elements of point-like structure;and

FIGS. 5A to C respectively show a diagrammatic illustration of a planview of an electroluminescent component according to the presentinvention with radiation decoupling elements of strip-like structure.

The exemplary embodiment of an electroluminescent component of FIG. 1concerns an LED chip 1, from which a large part of the electromagneticradiation 11 generated in the LED chip 1 is radiated in the mainradiating direction 6. The main radiating direction 6 is essentiallyoriented perpendicularly to the plane of main extent of the LED chip 1.

The LED chip 1 has a substrate 2, which has SiC or sapphire, forexample, in the case of an LED chip 1 based on Al_(x)Ga_(y)In_(1-x-y)Nwhere 0≦x≦1, 0≦y≦1 and x+y≦1. A Bragg reflector layer 3 is optionallyapplied on the substrate 2, and reflects back the light radiated fromthe emission zone 8 (explained later) in the direction of the substrate2. Such Bragg reflector layers 3 are known per se to the person skilledin the art and, therefore, are not explained in any further detail atthis point.

A plurality of radiation decoupling elements 4 arranged at a distancenext to one another are applied on said Bragg reflector layer 3. Asexplained in more detail further below with reference to FIGS. 4 and 5,said radiation decoupling elements may have, in plan view, a point-likestructure (with for example a circular, oval or polygonal lateralcross-sectional area) or a strip-like structure (with for example arectangular lateral cross-sectional area). The longitudinal central axes5 of the radiation decoupling elements 4 are oriented parallel to themain radiating direction 6 of the LED chip 1.

The radiation decoupling elements 4 have, on the optionally providedBragg reflector layer 3, an active layer stack 7 with an emission layer8A and a laterally delimited emission zone 8, which has at least oneelectroluminescent pn junction.

In a particularly preferred embodied, the active layer stack 7essentially comprises a plurality of doped and/or undopedAl_(x)Ga_(y)In_(1-x-y)N layers where 0≦x≦1, 0≦y≦1 and x+y≦1. Inprinciple, however, the structure according to the invention is alsosuitable for active layer stacks based on a plurality ofAl_(x)Ga_(y)In_(1-x-y)P or Al_(x)Ga_(y)In_(1-x-y)As layers where 0≦x≦1,0≦y≦1 and x+y≦1 or another suitable III-V or II-VI compoundsemiconductor.

The region between the contact element 9 and the emission zone 8preferably completely comprises Al_(x)Ga_(y)In_(1-x-y)N material, where0≦x≦1, 0≦y≦1 and x+y≦1, which is p-doped with Mg and/or Zn, particularlypreferably with Mg, and whose layer-parallel conductivity is so lowthat, in the event of current being injected into the chip, the currentspreading in the region between contact element 9 and emission zone 8 isless than 20 μm, in particular between 0.1 μm and 10 μm, so that thelateral cross-sectional area of the emission zone 8 is to the greatestpossible extent limited to the vertically projected area of the contactelement 9.

The radiation 11 generated in the emission zone 8 of the active layerstack 7 essentially emerges in a lateral propagation direction throughthe side areas 12 of the radiation decoupling elements 4 from the activelayer stacks 7 into the surrounding medium, such as, for example, aradiation-transmissive plastic capsulation (not illustrated) in whichthe LED chip is embedded and which comprises for example epoxy resin,silicone resin or another suitable reaction resin. Suitable electricallyinsulating and radiation-transmissive filling material may also besituated in the interspaces between the radiation decoupling elements 4.

Centrally arranged contact elements 9 are provided on the top sides ofthe radiation decoupling elements 4. Between the contact elements 9 andthe active layer stacks 7 of the radiation decoupling elements 4, acontact layer (not illustrated) may additionally also be applied atleast below the contact elements 9. As illustrated in FIGS. 4 and 5, theindividual contact elements 9 are connected to one another and to abonding pad 15 on the front side of the LED chip 1 by means ofelectrically conductive webs 14. Depending on whether a point-like or astrip-like structure of the radiation decoupling elements 4 is present,the contact elements 4 are formed as contact points or as narrow contactstrips.

A contact metallization 10 is applied over the whole area, by way ofexample, on that side of the substrate 2 which is remote from theradiation decoupling elements 4. However, a patterned contactmetallization having, by way of example, mutually isolated contact areasassigned in each case to a radiation decoupling element 4 may also beapplied here.

Between the radiation decoupling elements 4, a reflective layer 13 ispreferably applied on the substrate 2 or the Bragg reflector layer 3provided on the substrate 2, in order to reflect back the radiation 11which is decoupled from the radiation decoupling elements 4 and passesdownward to the substrate 2. This reflective layer 13 has advantageseven in the case of non-absorbent substrates 2 since it is possible toreduce reflection and transmission losses which occur upon entry intoand emergence from the substrate material.

The radiation decoupling elements 4 may be fabricated for example bymeans of whole-area epitaxial application of the Bragg reflector layer 3and of the active layer stack 7 to the substrate 2 and a subsequentphotolithography technique and etching. As an alternative, firstly amask layer is applied to the Bragg reflector layer 3, openingscorresponding to the structure of the radiation decoupling elements 4being etched into said mask layer and the active layers 7 subsequentlybeing deposited epitaxially into said openings. Finally, the mask layeris removed again by means of etching, for example.

The precise construction and the functioning of the radiation decouplingelements 4 of the electroluminescent component according to theinvention will now be described using various embodiments with referenceto FIGS. 2A to 2C.

In the exemplary embodiment illustrated in FIG. 2A, the emission zone 8in the active layer stack 7 is provided directly below the contactelement 9. While the height h of the active layer stack 7 is usuallypredetermined, the width b of the patterned radiation decouplingelements 4 is chosen, according to the invention, to be as small aspossible. In the case of the exemplary embodiment of FIG. 2A, the widthb of the active layer stack 7 preferably satisfies the condition0<(b+b′)/h<2 cot(α_(T))where b′ is the width of the contact element 9 which is dimensioned tobe significantly less than the width b of the radiation decouplingelement 4, and α_(T) is the critical angle of total reflection for theradiation 11 emerging from the active layer stack 7 into the surroundingmedium. For GaN, α_(T) is 37°, by way of example, so that the ratio(b+b′)/h should as far as possible be less than 2.65.

In some compound semiconductors, such as in p-dopedAl_(x)Ga_(y)In_(1-x-y)N for example, on account of a low mobility of thecharge carriers, the current spreading is so low that the emission zone8 essentially extends only to the vertically projected region of theemission layer 8 a below the contact element 9, i.e. the lateral extentand thus also the width of the emission zone 8 is, if at all, onlyinsignificantly greater than the lateral extent and thus the width b′ ofthe contact element 9. By virtue of the above-described dimensioning ofthe active layer 8, a maximum proportion of the light 11 radiatedlaterally from the emission zone 8 can be decoupled directly through theside area 12. A total reflection essentially does not take place at theside area 12.

Moreover, a large part, i.e. approximately cos(α_(T))=60%, of theradiation primarily generated by the emission zone 8 is radiated intothis angular range, so that this radiation can be decoupled into thesurrounding medium to the side area 12 directly, i.e. without priorreflection processes at the upper and lower boundary layers of theactive layer stack 7, which always also signify a reflection loss, andwithout further lossy passages through the emission layer 8 a. Inconventional systems, the radiation generated in the emission zones isusually reflected a number of times at the interfaces between activelayer stack and substrate or top side of the active layer stack andsurrounding medium before a decoupling is effected through the sideareas. Moreover, non-radiative losses as a result of surfacerecombinations can be avoided to the greatest possible extent accordingto the invention.

In contrast to the known components, such as, for example, the LED chipdisclosed in DE 199 11 717 A1, the present invention provides anelectroluminescent component which achieves a higher external efficiencyof the decoupling of radiation without additional structuring measuressuch as the introduction of a current aperture layer or an oxidediaphragm.

Even if the dimensioning of the radiation decoupling elements 4 does notlie within the limits specified above for optimum decoupling ofradiation, by virtue of the width b of the radiation decoupling elements4 which is to be chosen to be as small as possible, advantages may beachieved in any event in comparison with the conventional systems sincethere is a reduction at least of the number of reflection and absorptionprocesses connected with radiation losses before the decoupling from theradiation decoupling elements 4.

The same dimensioning of the radiation decoupling element 4 is chosen inthe case of the exemplary embodiment of FIG. 2, in which the emissionzone 8 is provided directly above the substrate 2 or the Bragg reflectorlayer 3 possibly present.

In other words, in this case, too, the width b of the radiationdecoupling elements should lie in the range0<(b+b′)/h<2 cot(α_(T))in order that a large part of the radiation generated in the emissionzone 8 is decoupled directly through the side areas 12 of the radiationdecoupling elements 4.

FIG. 2B illustrates a radiation-decoupling element 4 in which theemission zone 8 is not provided directly at the upper or lower edge ofthe active layer stack 7, but rather is arranged approximately in thecenter of the active layer stack 7. Based on the same consideration asfor the construction of the radiation decoupling element 4 asillustrated in FIG. 2A, the width b of the active layer stack 7 in thiscase should as far as possible satisfy the more stringent condition0<(b+b′)/h<cot(α_(T))in order to achieve the same effect.

The idea on which the present invention is based is, for a height hpredetermined by the active layer stack 7, to restrict the width b ofthe active layer stack in such a way that a largest possible proportionof the light radiated laterally from the emission zone 8 can bedecoupled directly through the side areas 12 since the angle at whichthe radiation impinges on the interface with the surrounding medium isless than the critical angle of total reflection. Therefore, the personskilled in the art will readily adapt the upper limit of the optimumdimensioning of the radiation decoupling element 4 between cot (α_(T))and 2 cot (α_(T)) if the emission zone 8 is arranged at an arbitraryheight position of the active layer stack 7 between the upper and thelower interface.

If the emission zone 8 in the active layer stack 7 does not lie directlybelow the contact element 9 as shown in FIG. 2A, a conical structuringof the radiation decoupling element 4 is advantageous, as is shown byway of example in FIG. 3.

In this exemplary embodiment, the side areas 12 of the radiationdecoupling elements 4 are formed as oblique etching side walls, thatside of the active layer stack 7 which faces the substrate 2 beinglarger than its side facing the contact elements 9. In the case of apoint-like structure of the radiation decoupling elements 4, this leadsfor example to a structure in the form of a truncated cone.

The heights h₁ and h₂, specifying the position of the emission zone 8within the active layer stack 7, are usually predetermined by theepitaxially applied layer 8 and, when added, produce the height of theradiation decoupling element 4 (h₁+h₂=h). The structure width b and thebase angle β of the oblique side areas 12 are then adapted in order toachieve an as far as possible optimum decoupling of light through theside areas 12 in such a way that the angular and side ratios preferablyagain satisfy the condition0<(b+b′)/h<cot(α_(T))

In this case, however, it must be taken into account that the criticalangle α_(T) of total reflection is related to the perpendicularconnection between the center point of the emission zone 8 and the sideedge 12.

Various possibilities for the configuration of the LED chip 1 will nowbe described with reference to FIGS. 4 and 5. In this case, FIGS. 4A and4B show, in plan view, exemplary embodiments with point-like structuresof the radiation decoupling elements, while FIGS. 5A to 5C illustrate,in plan view, various exemplary embodiments with strip-like structuresof the radiation decoupling elements.

The radiation decoupling elements 4 formed as cylinders or polyhedralhave a contact point 9 centrally in each case at their top side. Saidcontact points 9 are connected via electrically conductive webs 14 bothto one another and to a bonding pad 15 provided, by way of example, inthe center of the LED chip 1. In this case, the radiation decouplingelements 4 are positioned for example on the corner points of an(imaginary) hexagonal structure (FIG. 4A) or rectangular structure (FIG.4B). In FIGS. 4A and 4B, the light circles in each case indicate the topsides of the radiation decoupling elements with straight or conical sideareas.

In the case of strip-like structures of the radiation decouplingelements 4 these emerge for example radially from a bonding pad 15arranged in the center of the LED chip 1 and ramify further in regulargeometrical forms (FIG. 5A). For the sake of better clarity, only thecontact elements 9 and the corresponding connecting webs 14 areillustrated in FIG. 5; the radiation decoupling elements 4 which run ina strip-like manner and in each case run below the electrical connectingelements 9, 14 have been omitted.

As an alternative, the radiation decoupling elements 4 structured in astrip-like manner may also be positioned in a rectangular arrangement(FIG. 5B) or a hexagonal arrangement (FIG. 5C). This arrangement of theradiation decoupling elements 4 exhibits advantages with regard tosupplying all the radiation decoupling elements 4 with current.

1. An electroluminescent body (1), having a substrate (2); a pluralityof radiation decoupling elements (4) arranged at a distance next to oneanother on the substrate (2) and having a width (b) and a height (h),which in each case have an active layer stack (7) with an emission zone(8); and a contact element (9) on each radiation decoupling element (4),wherein the contact elements (9) have a width b′, which is less than therespective width b of the assigned radiation decoupling element (4); andthe width b of the radiation decoupling elements (4), for a given heighth, is chosen in such a way that substantially no light radiatedlaterally from the emission zone (8) is subjected to total reflection atthe side areas (12) of the radiation decoupling elements (4), but ratheris decoupled directly there.
 2. The electroluminescent body as claimedin claim 1, wherein the radiation decoupling elements (4) have astrip-like structure having the width (b).
 3. The electroluminescentbody as claimed in claim 1, wherein the radiation decoupling elements(4) have a point-like structure whose diameter corresponds to the width(b).
 4. The electroluminescent body as claimed in claim 3, wherein theradiation decoupling elements (4) are formed in cylindrical orpolyhedral fashion.
 5. The electroluminescent body as claimed in claim1, wherein the contact element (9) on the radiation decoupling elements(4) are electrically conductively connected to one another and to abonding pad (15) on the front side of the component (1).
 6. Theelectroluminescent body as claimed in claim 1, wherein the radiationdecoupling elements (4) taper in the direction away from the substrate(2) at least over a part of their height h, in particular are formedconically, and in particular their side facing the substrate (2) is ineach case larger than the side facing the contact elements (9).
 7. Theelectroluminescent body as claimed in claim 1, wherein the dimensions ofthe radiation decoupling elements (4) satisfy the condition0<(b+b′)/h<2cot(α_(T)) where α_(T) is the critical angle of totalreflection for the light emerging from the active layer stack (7) intothe surrounding medium.
 8. The electroluminescent body as claimed inclaim 7, wherein for the case where the emission zone (8) of the activelayer stack (7) in the radiation decoupling elements (4) is positionedbetween the side facing the substrate (2) and the side facing thecontact elements (9), the dimensions of the radiation decouplingelements (4) satisfy the condition0<(b+b′)/h<cot(α_(T)) where α_(T) is the critical angle of totalreflection for the light emerging from the active layer stack (7) intothe surrounding medium.
 9. The electroluminescent body as claimed inclaim 1, wherein a reflective layer (13) is provided on the substrate(2) between the radiation decoupling elements (4).
 10. Theelectroluminescent body as claimed in claim 1, wherein the regionbetween contact element (9) and emission zone (8) at least partiallycomprises a material whose conductivity is so low that essentially nocurrent spreading or a current spreading of at most 20 μm is effected inthe event of the operating current being injected vertically in theregion between the contact element (9) and the emission zone (8). 11.The electroluminescent body as claimed in claim 1, wherein the activelayer stack (7) has, at least between contact element (9) and emissionzone (8), a semiconductor layer made of Al_(x)Ga_(y)In_(1-x-y)N, where0≦x≦1, 0≦y≦1 and x+y≦1, the transverse conductivity of which is so lowthat essentially no current spreading or a current spreading of at most20 μm is effected in the event of the operating current being injectedvertically in the region between the contact element (9) and theemission zone (8).
 12. The electroluminescent body as claimed in claim11, wherein at most a current spreading of between 0.1 μm and 10 μm iseffected.
 13. The electroluminescent body as claimed in claim 11,wherein the semiconductor layer comprises Al_(x)Ga_(y)In_(1-x-y)N whichis p-doped in particular with Mg and/or Zn.
 14. The electroluminescentbody as claimed in claim 11, wherein the active layer stack (7) isfabricated altogether from semiconductor layers made ofAl_(x)Ga_(y)In_(1-x-y)N where 0≦x≦1, 0≦y≦1.
 15. The electroluminescentbody as claimed in claim 11, wherein the substrate (2) is a sapphiresubstrate.